The level of fine particles - specks of pollutants that can penetrate the lungs - in Hong Kong's air is among the worst in more than 500 cities and at least 20 times that of the cleanest metropolis.
Only seven cities of 565 surveyed by the World Health Organisation have a higher level than that found in Central, which also has the greatest concentration of larger particles among more than 1,000 cities.
The annual mean roadside reading of fine particles in Central - 36 micrograms per cubic metre - is exceeded only by Dakar in Senegal, Zabrze in Poland, Accra in Ghana, Kuwait City, Mexicali in Mexico, Antananarivo in Madagascar, and Ulan Bator in Mongolia.
The roadside reading in Central was used as a benchmark to compare with the cities in the WHO report, which did not make clear how many readings were taken in each city, but some were roadside or general.
Friends of the Earth said it was 'disappointing and shameful' that Hong Kong fared worse than developing cities. 'Despite being a top class world financial hub, people in Central are breathing third-world-class air,' said Jo Chan Chun-yim, assistant environmental affairs officer of the green group.
The rankings, released by the WHO in September, did not include Hong Kong because fine particles with a diameter less than 2.5 microns are not listed as a statutory air pollutant in the city.
But Friends of the Earth obtained data from the Environmental Protection Department showing the levels at the junction of Chater Road and Des Voeux Road Central was at least 20 times higher than the top ranking city, Whitehorse in Canada, with an annual average reading of 1.7. It was worse than Singapore with 19, Manila with 21, and Lima in Peru with 34.
Medical specialists have warned that the fine particles - a micron is one-millionth of a metre - can penetrate deep into the respiratory system and cause severe health risks.
In the same ranking exercise by the WHO for concentrations or larger particles, Hong Kong came 870 on a list of 1,100 cities.
Dr Lau Ngai-ting, from the environment division at the Polytechnic University of Science and Technology, said a single spot reading might not be indicative, while a comparison with another city might be too generalised. But this should not obscure the serious health concerns raised. 'Given the heavy traffic and people flow at this localised spot, there is definitely a health impact since many people will be exposed to the pollution,' he said.
He said poor ventilation and dispersion of pollutants might also contribute to the pollution that was believed to be primarily caused by diesel vehicles, especially those that were old and poorly maintained.
Friends of the Earth said even more frustrating was that the government was aiming to introduce a new fine particle standard not matching its status as an advanced metropolis.
The Environmental Protection Department proposes an annual mean standard of 35 micrograms compared with the WHO's 25, Thailand's 25, Singapore's 15 and Australia's much more stringent eight.
Even this modest standard has yet to be formally introduced even though the public consultation on updating the 24-year-old air quality objectives ended two years ago.
Chief Executive Donald Tsang Yam-kuen pledged earlier to get the task done this year.
'Donald Tsang is going to look like Pinocchio, with his lies stretching his nose so long that he will no longer be able to cover it with a mask to shield the air pollution,' Thomas Choi Ka-man, the green group's senior environmental officer said.
A department spokeswoman said the proposed standard, which referred to WHO guidelines, was a 'very challenging' one. She said new air quality objectives would be introduced in this government's term.
micrograms of fine particles per cubic metre have been recorded in a roadside reading in Central
- 20 times worse than in the cleanest city